The present invention relates to a transfer device for use in an electrophotographic image forming apparatus in which a toner image as formed on an image carrier is firstly transferred to an endless intermediate transfer belt and the toner image is secondly transferred from the intermediate transfer belt to a record medium such as a sheet of paper (hereinafter referred to merely as a sheet). The present invention relates in particular to a transfer device which controls transfer power to be supplied in first and second transfer operations.
Japanese Patent Application Laid-Open No. H10-039651 discloses a tandem-type full-color image forming apparatus having a semiconductive endless belt and a plurality of (e.g. four) image forming sections. The endless belt is installed rotatably, and the image forming sections each provided for forming a developed image of corresponding color are aligned along an outer circumference of the endless belt. This arrangement allows a full-color image to be formed in at least one full rotation of the endless belt.
There is also known a tandem-type full-color image forming apparatus using an intermediate transfer method. In the image forming apparatus, developed images for respective colors formed on photoreceptor drums as image carriers in respective image forming sections are accumulated on an outer circumferential surface of an endless belt (an intermediate transfer belt) and then transferred to a sheet, to form a full-color image.
More specifically, toner images are formed on the image carriers in the respective image forming sections, based on image data for the respective colors obtained by color separation from an original image. The toner images are firstly transferred from the image carriers to the intermediate transfer belt to be accumulated, or first transfer operations are performed. Then, the accumulation of toner images is secondly transferred from the intermediate transfer belt to the sheet, or a second transfer operation is performed.
Accordingly, the formation of a full-color image involves the first transfer operations performed in a plurality of, for example four, first transfer regions, and the second transfer operation performed in a second transfer region other than the first transfer regions. While following a loop travel path, the intermediate transfer belt passes through the first transfer regions and the second transfer region, in the order.
In the image forming apparatus using the intermediate transfer method, transfer power in full-color image formation is supplied to the intermediate transfer belt in the first transfer regions and in the second transfer region. The transfer power supplied to one of the transfer regions has undesirable effects on another transfer region positioned downstream thereof through the intermediate transfer belt, thereby preventing a predetermined transfer power from being supplied to the transfer region positioned downstream.
This is particularly true in case of the second transfer region being positioned immediately downstream of the first transfer region provided most downstream with respect to a traveling direction of the intermediate transfer belt with an aim to downsize the apparatus and achieve high-speed image formation. As a result, a toner image on the intermediate transfer belt cannot be transferred properly to a sheet.
Since a black toner image is generally transferred to the intermediate transfer belt in the first transfer region positioned most downstream with respect to the traveling direction, the transfer power supplied to the first transfer region interferes with the transfer operation in the second transfer region, in monochromatic image formation as well.
There have been proposed solutions to the foregoing problem, such as arrangement of first and second transfer regions at a longer distance from each other or use of an intermediate transfer belt with a higher resistance.
However, such arrangement of the first and second transfer regions causes an increase in size, and a decrease in image formation speed, of an image forming apparatus. Also, the intermediate transfer belt with a higher resistance requires a discharging device for each of the transfer regions, resulting in an increase in size and in manufacturing costs of the apparatus.
A feature of the present invention is to offer a transfer device that controls transfer power supply so that transfer power is timely supplied to the intermediate transfer belt in first and second transfer regions. Simple timing control allows the transfer device to avoid the undesirable effects of transfer power supplied to the respective transfer regions on the transfer operations performed in the other transfer regions. The transfer device thereby allows uniform transfer operations to be performed in the respective transfer regions and therefore constant high-quality image formation to be achieved, without an increase in size, or a decrease in image formation speed, of the image forming apparatus.